Determination of the volumetric flow rate of fluid within a conduit that is not directly accessible is typified by the determination of cardiac output, i.e., the volumetric flow rate of blood in the pulmonary artery. Various methods have been commonly implemented in cardiac output determination and various other methods and apparatus have been proposed. The hallmark method is that expounded by Adolf Fick in 1870, and subsequently developed coincident with modern cardiac catheterization techniques. Blood samples are obtained from the pulmonary artery and from a systemic artery, and their oxygen content is measured. The consumption of oxygen per unit time, under steady state conditions, is either assumed or measured. Cardiac output is then calculated by dividing the oxygen consumption by the difference between the measured arterial and mixed-venous oxygen content. Unfortunately, the method is complicated, cumbersome to use, and yields poorly reproducible results.
Dye-dilution methods also have been used widely but are not considered to be significantly more accurate or precise. Such methods employ an indicator dye which is injected as a bolus into a blood vessel. Thereafter, the changing concentration of indicator dye is measured at a downstream site and plotted versus time. Cardiac output is then computed by integrating a portion under the resultant curve. Dye-dilution methods are hampered by problems of cumbersome apparatus, loss of indicator dye, recirculation of indicator dye, and anatomic circulatory shunts.
The problems of indicator dye loss and recirculation essentially were solved by the thermal dilution modification introduced by Fegler in 1953. In this method, a bolus of cold solution or "dye" is injected into the blood vessel through a proximal port of a multiple lumen catheter, and the subsequent "dilution" or changing blood temperature with time is measured downstream through the use of a thermistor located on the distal end of the catheter. The resultant concentration-time variation is electronically integrated and cardiac output computed therefrom. A representative catheter for use in thermal dilution methods can be seen in U.S. Pat. No. 3,726,269, Webster, Jr. In its most widely used form, this type of catheter also has an inflatable segment or balloon which is used as a flotation device to facilitate positioning of the catheter in the pulmonary artery. Reference, for example, U.S. Pat. Nos. 3,995,623, Blake et al., 4,024,873, Antoshkiw et al., 4,105,022, Antoshkiw et al., and 4,329,993, Lieber et al. Thermal dilution methods have proven, however, to be no more accurate than the Fick method, require cumbersome apparatus, and yield information intermittently.
A plethora of additional methods and apparatus, all employing intravascular catheters or probes, have been proposed for cardiac output measurements. These include apparatus that determine cardiac output by measurement of the differential temperature that results from localized heating of the blood (U.S. Pat. Nos. 3,359,974, Khalil; 3,798,967, Gieles et al.); that use electromagnetic energy (U.S. Pat. No. 3,347,224, Adams); or that measure the conductivity of the blood (U.S. Pat. No. 3,896,373, Zelby). Yet another alternative technique determines cardiac output from balloon and arterial pressure signals obtained from a balloon catheter (U.S. Pat. No. 3,985,123, Herzlinger et al.).
Perhaps the most promising methods for cardiac output determinations are those utilizing a catheter bearing an ultrasonic transducer. Reference, for example, U.S. Pat. No. 3,430,625, McLeod, Jr., which discloses an intravascular catheter bearing a pair of ultrasonic transducers at its tip. The transducers are coupled with a Doppler circuit which applies a high-frequency electrical signal (typically in the mHz range) to one of the transducers so as to cause the transmission of ultrasonic energy therefrom. An electrical output signal from the other transducer, resulting from returns of the transmitted ultrasonic energy from the blood cells, is compared with the high-frequency electrical signal applied to the transmitting transducer to develop a Doppler signal that is representative of any frequency shift caused by relative movement between the blood cells and the catheter. The Doppler signal is therefore directly related to blood velocity. Improved apparatus of this type, using a single transducer for transmission and reception, can be seen in U.S. Pat. No. 3,443,433, Liston et al.
Blood velocity by itself is not sufficient to determine cardiac output because the concurrent effective internal cross-sectional area of the blood vessel must also be known. In the past, such determination of effective internal cross-sectional area has been accomplished by the use of transducer arrays and range-gating or signal-power ratio processing methods, all of which require intricate and meticulously constructed transducers as well as complex electronic processing units. Reference, in this regard, U.S. Pat. Nos. 3,542,014, Peronneau, 4,142,412, McLeod et al., 4,237,729, McLeod et al., 4,259,870, McLeod et al., and 4,232,373, Jackson et al. Although these methods are capable of providing superior accuracy and precision in the determination of cardiac output, the complexity and expense of the catheter and of the related signal processing apparatus has significantly limited their widespread commercial application.
It is therefore the principal object of the invention to provide a method for volumetric flow rate determination, such as cardiac output determination, which employs a simply and inexpensively constructed catheter bearing an ultrasonic transducer that can be discarded after a single use and which requires minimal signal processing in the necessary concurrent determination of conduit effective internal cross-sectional area.